This invention relates to anti-theft systems for vehicles and is of the type in which the normal brake system of the vehicle is caused automatically to effect a braking action in response to unauthorized use of that vehicle. The system involves mechanism whereby normal operation of the vehicle takes place under conditions in which the brake fluid flow path is characterized by one-way valve mechanism in parallel with a bypass for same so that, in effect, the one-way valve mechanism in ineffective. In the anti-theft operation of the system, the bypass is defeated so that the one-way valve mechanism controls flow of the brake fluid so that it will pass only to the brake slave cylinder with no, or substantially no, return flow.